This invention relates to a loudness control circuit in which, when the loudness of an input sound signal is low, the high frequency components and the low frequency components of the sound signal are increased in sound pressure, in order to compensate the frequency characteristic heard by a listener.
The frequency characteristic of sound which is audible to the human ear depends on the volume of the sound. As the volume of the sound decreases, it becomes difficult for a person to hear the low frequency components and the high frequency components of the sound. In order to solve this problem, acoustic devices such as stereophonic sound reproducing devices are provided with a loudness control circuit.
An example of a conventional loudness control circuit of this type is shown in FIG. 4. The circuit of FIG. 4 comprises a sound volume adjusting variable resistor 1 with center tap T (which divides the variable resistor into upper part 1a and lower part 1b), a high frequency bypassing capacitor 2, low frequency blocking capacitor 3, shunt resistor 4, and loudness switch 5. Loudness switch 5, together with other switches, is mounted on the operating panel of an acoustic device such as a stereophonic sound reproducing device.
In the loudness control circuit shown in FIG. 4, the sound volume of the output signal is controlled (increased or decreased) by sliding slider 1c of sound volume adjusting variable resistor 1. The loudness of the output signal is controlled by operating loudness switch 5. When loudness switch 5 is set to ON, the high frequency components of the input sound signal pass through high frequency range bypassing capacitor 2 and are applied to lower part 1b of variable resistor 1, thus being emphasized. On the other hand, although the low frequency components of the input sound signal tend to flow through the upper part 1a of the variable resistor 1 to the shunt resistor 4, they are blocked by the low frequency blocking capacitor 3. As a result, the low frequency components flow in lower part 1b of the variable resistor 1, thus being emphasized. Therefore, the sound reproduced is less attenuated both in high frequency range and in low frequency range as shown in FIG. 5, which illustrates the loudness effect provided by the loudness control circuit.
In the loudness control circuit, loudness switch 5 is a mechanical switch, while capacitors 2 and 3 and resistor 4 are provided on a printed circuit board. Hence, signal lines in which a sound signal flows must be connected between the contacts of loudness switch 5, capacitors 2 and 3, and resistor 4. This may lower the S/N ratio of the signal.
This difficulty may be eliminated by the following method. As shown in FIG. 6, the contacts of loudness switch 5 are replaced with electronic switching elements .alpha. and .beta. such as transistors or field-effect transistors. These electronic switching elements are arranged on the same printed circuit board with capacitors 2 and 3 and resistor 4 so that they are connected to the latter with the shortest conductors. In this circuit, only the means for providing on and off control signals for the electronic switching elements are installed on the operating panel.
FIG. 7 shows a transistor employed as the electronic switching element .alpha.. In a loudness "on" operation (with the loudness switch turned on), it is necessary to cause a DC bias current to flow between the base and the emitter of the transistor to render the latter conductive. The DC bias current is not related to the sound signal at all. This DC bias current flows in the sound volume adjusting variable resistor 1, thus lowering the S/N ratio of the sound signal. Furthermore, the switching operation for application of the bias current may result in the production of transient noises.
In order to eliminate these difficulties, the following methods are employed. In one method, shown in FIG. 8, only the electronic switching element .beta. for low frequency components is replaced with a transistor or the like. The electronic switching element .alpha. for high frequency components is eliminated and high frequency bypassing capacitor 2 is connected directly to center tap T of variable resistor 1, which may lower the S/N ratio. In another method, shown in FIG. 9, both high frequency bypassing capacitor 2 and electronic switching element .alpha. are eliminated, and only the electronic switching element .beta. for low frequency components is replaced with a transistor or the like.
However, these circuits are still disadvantageous in the following respects. In the circuit shown in FIG. 8, the high frequency bypassing capacitor 2 is fixedly connected. Therefore, whether the loudness switch is turned on or off, the high frequency components are emphasized in loudness at all times. In the case of the circuit shown in FIG. 9, the high frequency bypassing capacitor 2 is eliminated, and therefore the high frequency components cannot be emphasized in loudness.